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# A complete working Python program to demonstrate all
# stack operations using a doubly linked list
class Node:
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null
        self.prev = None  # Initialize prev as null
class Stack:
    >>> stack = Stack()
    >>> stack.is_empty()
    >>> stack.print_stack()
    stack elements are:
    >>> for i in range(4):
    ...     stack.push(i)
    >>> stack.is_empty()
    >>> stack.print_stack()
    stack elements are:
    3->2->1->0->
    >>> stack.top()
    >>> len(stack)
    >>> stack.pop()
    >>> stack.print_stack()
    stack elements are:
    2->1->0->
    def __init__(self):
        self.head = None
    def push(self, data):
        """add a Node to the stack"""
        if self.head is None:
            self.head = Node(data)
        else:
            new_node = Node(data)
            self.head.prev = new_node
            new_node.next = self.head
            new_node.prev = None
            self.head = new_node
    def pop(self):
        """pop the top element off the stack"""
        if self.head is None:
            return None
        else:
            temp = self.head.data
            self.head = self.head.next
            self.head.prev = None
            return temp
    def top(self):
        """return the top element of the stack"""
        return self.head.data
    def __len__(self):
        temp = self.head
        count = 0
        while temp is not None:
            count += 1
            temp = temp.next
        return count
    def is_empty(self):
        return self.head is None
    def print_stack(self):
        print("stack elements are:")
        temp = self.head
        while temp is not None:
            print(temp.data, end="->")
            temp = temp.next
# Code execution starts here
if __name__ == "__main__":
    # Start with the empty stack
    stack = Stack()
    # Insert 4 at the beginning. So stack becomes 4->None
    print("Stack operations using Doubly LinkedList")
    stack.push(4)
    # Insert 5 at the beginning. So stack becomes 4->5->None
    stack.push(5)
    # Insert 6 at the beginning. So stack becomes 4->5->6->None
    stack.push(6)
    # Insert 7 at the beginning. So stack becomes 4->5->6->7->None
    stack.push(7)
    # Print the stack
    stack.print_stack()
    # Print the top element
    print("\nTop element is ", stack.top())
    # Print the stack size
    print("Size of the stack is ", len(stack))
    # pop the top element
    stack.pop()
    # pop the top element
    stack.pop()
    # two elements have now been popped off
    stack.print_stack()
    # Print True if the stack is empty else False
    print("\nstack is empty:", stack.is_empty())

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